My "go to" general resource for medieval Welsh genealogy is Carl Boyer, 3rd, Medieval Welsh Ancestors of Certain Americans. In his entry for Thomas ap Llywelyn ab Owain ap maredudd, #74 on page 316, Boyer shows Thomas child, Elen, "by first wife, given by Bartrum," Eleanor ferch Maredudd. Boyer goes on to note that the child Maredudd is by Thomas' second wife, and that the mother of children Margred, Owain, and Eleanor is not specified in Bartrum. Bartrum can be checked on the internet (others are better are this than I), but Bartrum claims simply to have compiled and organized the medieval Welsh pedigrees, which are notoriously long on names and short on dates. Without the controversy, this would probably be enough for me to add Elen's mother, but since there is a controversy, it would be important to check Bartrum and which actual manuscripts Bartrum is citing in this case.